High speed tubular belt conveyor and system and method for making

ABSTRACT

A high speed, normally closed tubular belt conveyor system operable over long distances and through relatively short radius horizontal and vertical turns, is described. The belt conveyor is prestressed at fabrication to provide a normally closed tubular configuration. Profiled end pulleys are provided which open the belt for loading and unloading with minimum strain and distortion. A distributive drive system provides reduced longitudinal tensile stresses in the belt and facilitates high speed operation. A feeder-accelerator system loads granular-type material into the tubular belt at a rate and velocity which are conformed to the capacity and velocity of the belt.

This is a continuation of application Ser. No. 08/584,985, filed Jan.11, 1996, which is a continuation of application Ser. No. 08/324,133,filed Oct. 17, 1994, which is a continuation of application Ser. No.08/195,596, filed Feb. 14, 1994, which is a continuation of applicationSer. No. 07/737,586, filed Jul. 25, 1991, which is a continuation ofapplication Ser. No. 07/610,458, filed Nov. 8, 1990, which is acontinuation of Ser. No. 07/239,528, filed Aug. 31, 1988, all nowabandoned, which is a continuation of application Ser. No. 06/799,928,filed Nov. 20, 1985, now U.S. Pat. No. 4,823,941.

BACKGROUND OF THE INVENTION

The present invention relates to tubular belt conveyors and, inparticular, to a prestressed, self-closing tubular conveyor belt and tothe components and to the overall construction of a high speed conveyorsystem which uses such a tubular belt.

In general, the term tubular belt or tubular conveyor belt refers to abelt which typically has a relatively narrow width compared to itslength and which is rolled or formed along its width into a closed tubewith overlapping longitudinal edges. Preferably, when used in materialtransporting systems, such tubular belts are formed as endless conveyorsin which pourable material is loaded at one point and dispensed at asecond point.

For a number of reasons, conventional tubular conveyor belts have notfulfilled their potential for use in material transport. First,conventional tubular belts require cumbersome external guides, such asradially arranged rollers or funnels or stiff outer tubes, to form andmaintain their tubular configuration and to keep the belt edges closed.In addition, these tubular belt conveyors are subject to twisting,despite the use of external shaping devices. If the joint formed by theoverlapped belt edges is twisted from the preferred vertical position toa downward-facing position on the lower half of the tube, the belt edgesmay separate, allowing spillage of the bulk material carried by theconveyor.

Conventional tubular belt conveyor systems use a lumped drive systemcomprising a cylindrical drive pulley at one end of the conveyor belt, asecond cylindrical stretching pulley at the opposite end of the belt,and a set of supporting pulleys or other supports along the lines of thesystem. This construction has many of the known disadvantages of flatbelt conveyor systems. That is, the use of the single, lumped drive topull the entire conveyor line results in extremely high longitudinaltensions in the belt, especially on ascending slopes. The maximumconveyor length is limited by the tensile strength of the conveyor belt.The high tensile stresses in the belt require heavy reinforcement, andany attempt to lengthen the conveying lines requires even greater,heavier and typically more rigid and expensive reinforcement. Inaddition, the high torque capacities required of lumped drive systems topull such heavy and perhaps heavily reinforced conveyor lines usuallyrequires the installation of a speed reduction transmission, whichresults in additional energy losses.

Furthermore, high speed loading and unloading of a normally closed highspeed tubular belt conveyor is difficult. The normally closed belt mustbe opened and closed for both loading and unloading. In addition,efficiency in the conveying process requires high conveyor belt speedsand requires that the material be loaded through the opening onto thefast moving conveyor belt, transported by the belt, and off-loaded, allwithout spillage of material.

In short, the construction of conventional tubular belt conveyor systemslimits their speed and length, limits the number and degree of turns insuch systems, and makes difficult high speed efficient loading of suchsystems.

SUMMARY OF THE INVENTION

In view of the above discussion, it is one object of the presentinvention to provide a high speed tubular belt conveyor system which iscapable of operating at high speeds over long, essentially unlimiteddistances, and, where necessary, using a multiplicity of conveying lineturns.

It is another object of the present invention to provide a tubular beltconstruction which is constructed with prestress to inherently provide aclosed tubular configuration without the use of external locatingdevices.

It is also an object of the present invention to provide a distributeddrive system for a tubular belt conveyor in order to reduce the tensilestresses in the belt and to reduce the drive capacity requirements andto thereby permit the conveyor system to be formed to essentially anylength.

It is an associated object of the present invention to provide a tubularbelt conveyor construction and a drive and support system constructionwhich permit incorporating a multiplicity of horizontal and verticalturns as well as extended length and high speed capabilities into suchconveyor system.

It is an additional object of the present invention to provide a systemfor efficiently loading a high speed tubular belt conveyor, with theloading rate and velocity of the fed material corresponding to thecapacity and velocity of the moving conveyor belt.

It is still another object of the present invention to provide load andunload pulleys for a high speed tubular belt conveyor system which openthe normally closed belt with a minimum of strain and distortion.

The above and other objects of the present invention are achieved in aconveyor belt which is prestressed along its transverse width to providean openable, closed tubular configuration. In one aspect, the beltcomprises elastic inner and outer layers; which are joined together sothat the inner layer is in tension along its width and the outer layeris in compression along its width, with the result that internal bendingmoments maintain the belt in a normally closed tubular configuration. Inone embodiment the desired combination of compression and tension isprovided using elastic inner and outer layers having unstressed widthswhich are, respectively, less than and greater than the nominal beltwidth.

In an alternative embodiment, the belt includes an array ofcircumferential spring bands whose inherent spring action normally bendthe belt to a substantially tubular configuration and through an angleequal to or greater than 360°.

In still another aspect, the belt includes means for reinforcing thestiffness thereof along a selected diameter. The reinforcing meanscomprises a pair of relatively thick, longitudinally extending beltsections at said diameter and may include longitudinally extendingreinforcing bands. This construction imparts stiffness to the belt alongthe given diameter and provides structural integrity for maintaining thenormally closed tubular configuration, yet permits relatively easybending (turning) of the belt in directions generally transverse to thediameter. Such thick sections may also be used to maintain the alignmentof the belt relative to a guide or to a drive roller.

In another aspect, the present invention relates to a drive roller meansfor driving the adjacent branches (loaded and empty) of a tubularconveyor belt, comprising a drive roller mounted between the loaded andempty branches for rotatably engaging and driving the branch; and atleast one idler roller which is supported at the opposite side of theloaded conveyor branch from the drive roller for maintaining engagementof the drive roller with the loaded branch. Alternatively, idler rollersmay be mounted at the outside of both the loaded and empty branches formaintaining the drive roller in engagement with both the loaded branchand the empty branch of the tubular belt. In a preferred embodiment, anumber of such drive rollers are used to define and drive the conveyorbelt through vertical turns.

In still another aspect, the invention relates to a conveyor pulley foropening a normally closed tubular belt conveyor. The ratio r/R of theedge radius to the center radius of the pulley is selected tosubstantially equalize the paths traversed by the edge and centerregions of the belt around the pulley and thereby substantially decreasethe stress differential in the edge and center regions of the pulley. Inanother aspect, the pulley comprises a plurality of independent rotatingsections mounted on a common axis for adapting the angular velocity ofthe pulley sections to the tangential velocities of the associated beltsections to thereby substantially decrease differences in longitudinaltension across the width of the belt.

In another aspect, the present invention relates to a bunker fordispensing granular-type material onto a moving conveyor belt at adispensing flow rate and velocity that are conformed to the capacity andvelocity of the conveyor belt. The bunker comprises frame means; anupper bunker mounted to the frame for holding a quantity of the materialand having an opening in the bottom for dispensing the material; closuremeans for varying the size of the opening to conform the dispensing flowrate to the capacity of the moving conveyor; a plurality of funnelssupported by the frame between the bunker and the conveyor in alignmentwith the path of the dispensed material; means for varying the verticalposition of the funnels so that the funnels cooperatively conform thecross-section of the material flow to that of the conveyor; and a lowerfunnel aligned with the moving conveyor for conforming the direction ofthe material flow to the direction of the conveyor movement.

Additional objects and features of the invention will be evident fromthe following description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematized overview of one embodiment of the high speedtubular belt conveyor system of the present invention;

FIGS. 2 and 3 depict a construction for and method of prestressing andjoining the tubular belt of the present invention;

FIG. 4 is a transverse cross-section of a preferred embodiment of thelaterally prestressed tubular belt of the present invention formed bythe method of FIGS. 2 and 3;

FIGS. 5 and 6 schematically depict transverse cross-section and sideelevation views of reinforcement band means for the tubular belt of thepresent invention;

FIG. 7 schematically depicts a transverse cross-section view ofalternative reinforcement band means that provides increased verticalstiffness;

FIG. 8 is a transverse cross-section view of an embodiment of my tubularbelt, shown laid out in a flat configuration, which can incorporate thereinforcement means of FIG. 7;

FIG. 9 is a transverse cross-section of the tubular belt of FIG. 8 shownin its normally closed carrying configuration;

FIG. 10 is a transverse cross-section view of the tubular conveyor beltshown in FIG. 4, partly in schematic form, and showing a preferredembodiment of a drive roller for both branches of the belt;

FIG. 11 is a transverse cross-section of the tubular conveyor belt shownin FIG. 9, partly in schematic form, and showing a drive roller for bothbranches of the belt;

FIG. 12 is a transverse cross-section, partly schematic, of a one-rimdrive roller arrangement for the load branch of a tubular conveyor beltmodified to include a longitudinal drive groove of the type shown inFIG. 9;

FIG. 13 is a front elevation view of the feeder-accelerator tower ofFIG. 1, shown positioned for maximum productivity;

FIG. 14 is a side elevation view of the feeder-accelerator tower of FIG.1, shown positioned for reduced productivity;

FIGS. 15 and 16 are front elevation views of alternative embodiments ofthe feeder-accelerator tower of the present invention, employingtelescoping tube enclosures and hydraulic jacks, respectively;

FIG. 17 is a schematic side view depicting a normally closed tubularconveyor belt passing over an end opening pulley for loading orunloading;

FIG. 18 is a plan view of the unwinding belt of FIG. 17; and

FIG. 19 depicts the end pulley of FIGS. 17 and 18, shown partly insection for the purpose of illustrating the preferred segmentedconstruction of the pulley.

DETAILED DESCRIPTION OF THE INVENTION Overview

FIG. 1 is an overview of a high speed endless tubular belt conveyorsystem 10 that is constructed in accordance with the objects andprinciples of the present invention. FIG. 1 is divided into three.parts. Section 1A is a side elevation view showing the loading end ofthe conveyor system 10, the associated feeder-accelerator tower 11, andthe relatively small radius, vertical belt turn 12 that is used toposition the belt for loading. Section 1B is a perspective view of anintermediate section of the tubular conveyor belt system 10 whichillustrates the manner of implementing horizontal turns such as 13 and14. Section 1C is also a side elevation view, in this case of theopposite, unloading end of the tubular belt conveyor system 10. Section1C illustrates vertical turns 16 and 17 that are used to position thebelt for unloading.

As shown in Section 1A of FIG. 1, a transporter 18, which typically isnot part of the the present invention, can be used to load granular-typematerial into a bunker 19 of the feeder-accelerator tower 11.Alternatively, the output or unloading end of one conveyor system 10 canbe used to load the bunker 19 of a second conveyor system 10. A steadystream of the material is dispensed from the bunker 19 (see FIG. 13) andfalls down the feeder tower 11 through a series of movable funnels suchas 21 and 22 and then via a fixed funnel 23 into an opening 24 which isformed in the normally closed tubular belt 15 by the tension applied byend pulley 26. The movable bunker 19, movable funnels 21 and 22 andfixed funnel 23 cooperatively match the rate (volume/time) and velocity(speed and direction) of the material flow to those of the moving belt15. At pulley 26F the belt 15 is unwound or opened by tensile forces,and then is wound back into its normally closed tubular configuration byits internal prestress forces for the purpose of carrying the materialto the unloading station, shown in Section IC of FIG. 1. There the beltis unwound or opened by the tensile forces applied by a similar endpulley 27 to unload the material into the destination bunker 29.

The tubular belt 15 comprises the loaded branch 15A, which carriesmaterial from the loading point at pulley 26 to the unloading point atpulley 27, and an empty branch 15B, which preferably is adjacent andparallel to the load branch 15A. Alternatively, both branches 15A and15B can be load-carrying.

FIG. 1 also illustrates various structures which can be used fordriving, supporting and turning the tubular belt 15. The loaded andempty branches 15A and 15B are driven in opposite directions by driverollers 31 which are spaced along the conveyor lines and are constructedas described below for driving both branches of the line in oppositedirections. The distances between the drive rollers 31 may vary withvarying vertical slopes of the conveyor line. Conventional supportrollers 32 are used to support the loaded branch 15A at intermediatepoints between the drive rollers 31. In a preferred arrangement,horizontal turns such as those shown at 13 and 14 are defined byentrance and exit drive rollers 31 and by a number of intermediatehorizontal guide rollers 33 which may be constructed similar to oridentical to the vertically oriented support rollers 32. The number ofguide rollers 33 per turn depends upon the radius of the turn and thesize (and weight) of the conveyor lines.

Two interchangeable vertical turn constructions are shown. The firsttype is illustrated at load turn 12 and vertical turn 16. This type ofvertical turn uses a pair of spaced outer drive rollers 31 (which driveboth branches 15A and 15B) and one or more intermediate drive rollers 34(for driving the loaded branch 15A only). The second vertical turnconstruction, shown at unload turn 17, uses only drive rollers 31 (forboth branches) . The various drive rollers 31 and 34 and support andguide rollers 32 and 33 are mounted either on conveyor frames or girdersor to a structural framework of a building or to other suitable supportstructures. This supporting framework is not part of the invention andis not shown in FIG. 1.

The system 10 of FIG. 1 illustrates the situation in which material isloaded from the feeder-accelerator tower 11 onto the loaded branch 15Aof the tubular belt at a steeply inclined downward slope, levels out toa generally horizontal path which is routed in different directions byturns 13 and 14, then is routed through turn 16 into a vertical ascentand turned at 17 for unloading by end pulley 27. More generally, thedepicted features enable the conveyor line to be turned easily in anydirection needed, including vertical and horizontal directions, so thatmaterial can be loaded and transported over steep up and down slopes andaround various obstacles, to the unloading point 27. All conveyor lineturns have drive rollers 31 for both branches at the end points of theturn radius. The actual number of drive rollers 31 and 34 (at turns andalong straight sections), and the distances between drive rollers arefunctions of the desired conveying speed, material density, tubediameter, belt thickness and strength, internal belt prestress forcesand the nominal inclinations and turn radii of the conveying line. Driverollers are stationed at the loading and unloading points. Typically,vertical or steeply sloped sections (Section 1C) use relatively closelyspaced drive rollers without support rollers 32. In conforming theloading rate and direction of the loaded material to the conveyor belt15, the feeder-accelerator tower 11 enables efficient high speedoperation of the tubular belt conveyor system 10.

Tubular Belt 15 and Methods of Construction

The tubular belt 15, alternative embodiments of the belt and methods ofconstructing the various embodiments are depicted in FIGS. 2 through 9.As mentioned, a primary advantage of the belt 15 and its describedalternative embodiments is the inherent ability to maintain a tubularconfiguration. This configuration results from stresses that areincorporated into the belt during its fabrication. The prestressedforces are developed by using a multi-layer belt construction of two ormore layers in which the layers are prestressed then joined together asby gluing or welding, or by reinforcing the belt with a prestressedflexible spring-like carcass, or by a combination of these twoapproaches.

FIG. 4 depicts the operational tubular configuration of a presentlypreferred embodiment 15 of my tubular belt. The belt 15 is constructedas a two-layer composite comprising an inner elastic layer 36 and anouter elastic layer 37. The longitudinal edges of the belt arefabricated, respectively, as a tongue 38 and as a mating groove 39. Thesidewalls of the groove 39 are slightly wedged out (FIG. 3) , so thatwhen the belt is wound into its normally closed tubular configuration bythe inner prestress forces, tongue 38 locks into groove 39. The innerprestress forces that wind the belt into the tubular configuration aremade sufficiently strong during the fabrication process to normally bendthe belt through ≧360° so that the edges overlap. As a result, the beltedges at the joint are kept under compression and tightly closed duringoperation, including high speed loaded operation. The lateral stiffnessof the tubular belt 15 is substantially equal in all radial directions.

FIGS. 2 and 3 illustrate schematically my presently preferred method offabricating the two layer belt 15 of FIG. 2. As shown, the originalwidth of the outer layer 37, before prestressing, is greater than thenominal width of the belt 15. The original unstressed width of the innerlayer 36 is smaller than the belt width. Initially a prestressingtensile force T, uniformly distributed laterally over the entire lengthof inner layer 36, is applied to the edges of the inner layer, tostretch that layer laterally until its width coincides with that of theouter layer 37. This stretching is illustrated in phantom in FIG. 2.Next, the two layers 36 and 37 are joined together, as by applyingpressure as indicated schematically by arrows C, and are joined togetherby gluing or welding to form the composite two-layered belt 15. See FIG.3. Then, the tensile force T and the pressure C are removed, allowingthe released belt to shrink to its normal tubular configuration shown inFIG. 4. This tubular configuration is created and maintained by internalbending moments, distributed over the entire lateral cross-section ofthe belt, which result from the reactive lateral tensile stress in theinner layer 36 and the reactive lateral compressive stress in the outerlayer 37.

As one example, the layers 36 and 37 of the belt 15 can comprise anelastic material such as latex. To form the belt 15, the outer layer 37is laid flat on the lower platen of a press. The inner layer 36 isstretched laterally by attaching vises to its outer edges and applyingan outwardly directed tensile force T to stretch the inner layer so thatit coincides in width to the outer layer 37. Adhesive such as bargecement is then applied to the facing side of one or both of the innerand outer layers. The layers are pressed together by the press withforce C, and the tension T is maintained, until the adhesive dries. Forinner and outer layers 36 and 37, 1.5 mm thick and 60-80 mm wide,exemplary values of the tensile force T and compressive force C are 0.35kg/cm and 1.0 kg/cm², respectively.

Sufficient prestress forces for creating ending moments which wind thebelt are incorporated into the belt 15 to maintain the edges of the beltlocked under compressive force at the joint 40. This compressive force,P_(c), is defined as

    P.sub.c ≧P.sub.i +P.sub.r,

where P_(i) is the tensile force in a tube subjected to a maximuminternal outward material pressure, accounting for materialcharacteristics, such as density and angle of internal friction, andalso for nominal inclinations of the conveying line and dynamic effects;and

P_(r) is the reopening force at conveyor turns.

This compressive force can be computed as a tangent force in acylindrical membrane shell, subjected to global bending with apredefined curvature (equal to the nominal curvature of conveyor turns).

The compressive force acting at the joint formed by the two edgesensures a tight joint at any position along the belt and at any locationalong the conveyor line.

The above-described tubular construction is much more rigid in thevertical plane than an open belt. However, the lateral stiffness of thetubular belt is significantly lower than that of an open belt in thehorizontal direction. This provides an opportunity to arrange the turnsof the conveyor line in any direction with using relatively small radiiturns.

FIGS. 5 and 6 depict spring reinforcement means 45. The springreinforcement assembly 45 can be joined to a single or multiple layeredbelt or between the layers of the belt during its fabrication. Thereinforcement carcass typically comprises thin spring steelcircumferential bands 46 whose springing effect causes them, whenreleased, to bend to an angle greater than 360°. See the schematicconfiguration shown in FIG. 5. The bands 46--46 are connected to acentral rib element or spine 47. The spine 47 resists tension, but itsresistance to compression and bending and stiffness are essentiallynegligible. The reinforcing assembly 45 is completed by edge ribs 48that are similar to the central rib or spine 47.

When the spring reinforcing assembly 45 is incorporated in belts such as15, the tubular configuration of the prestressed belt can be provided bythe spring assembly 45 alone, or by the spring assembly in conjunctionwith the prestressed composite belt construction of FIGS. 2 and 3. Itshould be noted that the reinforcing carcass or spring assembly 45 canbe made of spring steel or of other strong flexible materials thatpossess "memory" as to their original shape. The spring reinforcementassembly is joined to the belt by lamination. For example, thereinforcing carcass can be positioned between the belt layers 36 and 37and laminated to these layers during the assembly depicted in FIGS. 2and 3.

For a conventional conveyor system using a flat belt, the idler rollersare spaced to limit the vertical deflection of the loaded branch of thebelt. Due to the catenary response of the belt, the longitudinal tensionin the flat belt reduces the deflections from the desired path ofmovement. In contrast, in a conveyor system using the tubular belt 15,the longitudinal tensile force is drastically reduced by the distributeddrive scheme and is of essentially no significance. This removes therestrictions on the length of the conveyor belt. At the same time, thevertical stiffness of the tubular configuration is essential in that italone, and not longitudinal tensile force, determines the distancesbetween the support rollers which serve as the gravity support for thecontinuous tubular beam. As a result, when conveying distances are verylong and the line is substantially horizontal, it is useful to make thebelt more rigid in the vertical plane without increasing its horizontallateral stiffness. This reduces the number of support rollers requiredwhile preserving the ability to incorporate horizontal turns anddetours.

FIGS. 7, 8 and 9 depict an alternative embodiment 50 of my tubular beltthat has this desirable combination of horizontal flexibility andincreased vertical rigidity or stiffness. The belt 50 includesrelatively thick, longitudinal top and bottom stiffening pads or strips51 and 52. For additional stiffness, and optionally, a spring biasingassembly 45, FIG. 6, or 55, FIG. 7, can be incorporated into the beltbetween the inner and outer belt layers 36A and 37A.

As shown in FIG. 7, the assembly 55 is an alternative embodiment of theassembly 45, shown in FIGS. 5 and 6, and incorporates a center ribelement 47A and an edge rib element 48A in place of the ribs 47 and 48used in the assembly 45. Each of the elements 47A and 48A preferablycomprises a flexible mesh of metal or other suitable material, or a pairof (or several) longitudinally-extending flexible tubes, also of metalor other suitable material. The stiffening ribs 47A and 48A are mountedbetween the belt layers 36A and 37A with their long transverse sideoriented vertically (that is, along the diameter of the desireddirection of stiffening). This enhances the vertical stiffness whileretaining horizontal flexibility. See FIGS. 7 and 9. Preferably, thisinherent spring action of the assembly 45 is sufficient to bend theassembly (prior to mounting to the belt) through at least 360°, andpreferably greater than 360°. As a consequence, when the spring assemblyis mounted to/in the belt 50, the springing action urges the belt edges38A and 39A to overlap. This provides secure locking of the belt edgesat the joint 40A, FIG. 9, with the ribs 47A and 48A aligned along thedesired diameter. Here, the desired diameter is vertical and the bands46A and 47A provide increased vertical stiffness (and resistance todeformation), while retaining lateral flexibility and, thus, smallradius turn characteristics.

Distributed Drive Method and System

Conventional flat belt endless conveyor systems drive the belt via oneof two end pulleys, using friction between the belt and the drivepulley. Increasing the length of the conveyor lines increases the powerand friction required to drive the conveyor belt. Friction can beincreased by increasing the belt tension. However, high belt tensileforces; not only require a heavily reinforced, expensive beltconstruction, but also limit the maximum length of even reinforced flatbelt conveyors to several hundred yards. In addition, large diameter endpulley drums are required for long conveyor runs, both because of theneed for a large friction surface and because the thick, heavilyreinforced belts cannot be bent through small radius turns. Furthermore,for a given conveying speed, increasing the diameter of the pulley drumlowers the rotational speed of the pulley, which requires a highertransmission gear ratio and is less efficient.

My distributive drive and the drive components used in driving thetubular belt conveyor eliminate the above shortcomings.

One preferred embodiment 31 of the driving device of my invention isshown in FIG. 10. The driving device 31, i.e., drive roller system 31,is mounted to means 61 such as the conveyor framework or girders orassociated building frame or girders. The driving device 31 comprises adrive idler 62, which is mounted between the loaded branch 15A and theempty branch 15B of the conveyor belt, and two satellite idlers 63--63,which are mounted on the opposite (outer) sides of the two conveyorbranches.

The drive idler 62 comprises an electric motor which has its stator 64fixed on a hollow shaft 66 which, in turn, is fixed to the framework 61.Rotor 67 is the coil of the electric motor and is rotatably mounted onthe shaft 66 on bearings 68. The motor wiring (not shown) isconveniently routed along the inside of the hollow shaft 66.

The satellite idlers 63--63 are mounted for rotation on axles 71 bybearings 69. Preferably, each axle is mounted to the frame 61 and movesonly up and down, in the z direction. For example, flanges (not shown)may be mounted at each end of the axle 71 at slots (not shown) formed inthe frame 61 to position the axle for vertical movement along the slots.Preferably, each satellite idler at axle 71 is also connected to thefixed drive idler shaft 66 through a flexible bond. As shown in FIG. 10,in one embodiment, this flexible bond is provided by a pair of springs72--72 which are mounted to and extend between the fixed hollow idlershaft 66 and the vertically movable satellite idler axle 71, one each onopposite sides of the conveyor branch and associated idlers. Tension inthe springs can be controlled by tightening or loosening retaining nuts73. The flexible bonds serve to press the conveyor branches 15A or 15Bfirmly against the drive idler 62 for driving the branches of theconveyor. In addition, rubber rings 74 can be mounted on the rims 76 ofthe drive idler rotor or barrel 67 to increase the friction between therims of the drive idler barrel and the tube belts.

In operation, when the rotor 67 is rotated, the drive idler rims 76engage and rotate the loaded and empty branches 15A and 15B to move thebranches in opposite directions with equal velocities. The driving forcedepends upon the amount of friction generated between the barrel orrotor 67 and the belt branches 15A and 15B. This friction is directlyproportional to the normal component of pressure at the contactsurfaces. Therefore, the driving force can be controlled by tighteningthe flexible connection or bond provided by the spring elements 72. Itwill be readily appreciated that the flexible bond can be provided byelements such as pneumatic, hydraulic, or electromagnetic cylinders ordevices. In addition, the drive system 31 can comprise more than onedrive idler. On steeply descending slopes, the drive system 31 may beoperated in the reverse regime, that is, it may be driven by the beltrather than vice versa, to generate electricity and partially recuperatethe total energy consumption.

FIGS. 11 and 12 depict alternative embodiments 31A and 34A of thedriving devices. Embodiment 31A, FIG. 11, utilizes the belt 50, FIGS.7-9. The reinforcement pads 51 and 52 are positioned between the rims 76of the drive idler 62 and between the rims 77 of the satellite idlers 63to guide the tubular conveyor belt and prevent twisting of the load andreturn branches.

Load branch driving device 34A shown in FIG. 12 also utilizes thereinforced tubular belt 50 and utilizes the idler rims to guide thebelt. In this embodiment, the drive idler 62A is formed with only asingle rim 76, while the satellite idler 63A is also formed with asingle rim 77. The rims 76 and 77 engage a pair of longitudinal grooves79 and 81 formed in the reinforcement pads 51 and 52 to provide theguiding and twist-prevention functions. The flexible bond is provided byhydraulic jacks 82. Of course, the driving device 34A can be constructedas a two satellite idler device.

Feeder-Accelerator Tower

Conventional conveyor loading approaches are unsuitable for loading fastmoving conveyor belts because of the tendency to destroy the belt and toscatter and destroy the material, as well as to only partially load theconveyor. My feeder 11 avoids such shortcomings and provides anefficient non-destructive approach for loading tubular belt conveyorsmoving at high speeds (conveying speeds of up to 40 mph and above).These qualities are achieved by conforming the material loading rate tothe capacity of the moving conveyor and by converging the velocityvector of the loaded material with the velocity vector of the tubularbelt at the loading belt opening 24, FIG. 1.

Referring to FIGS. 1 and 13, the feeder-accelerator tower 11 (or,simply, feeder 11) comprises a support structure such as a tubular frametower 83 on which bunker 19 and funnels 21 and 22 are slidably mountedfor vertical movement. Funnel 23 is fixedly mounted on frame 83. Thefunnel 23 includes a curved sleeve 84 which fits into the load opening24 formed in the conveyor belt 15 by the pulley 26. The funnel sleeve 84is aligned with the direction of movement of the conveyor at the openingpoint to guide the material dispensed from the loader/transporter 18 inthe direction of travel of the load branch 15A of the tubular conveyorbelt.

The rate and velocity at which the material dispensed from the loader 18is loaded onto the belt branch 15A is controlled by lifting device 86which is mounted toward the top of the frame or tower 83. The liftingdevice 86 includes a number of pulleys which are rotatably mounted on anaxle 87 which is driven reversibly by motor 88. Bunker 19 is suspendedby cables 89 attached to pulley 91 for raising and lowering. Funnel 21is similarly suspended by cables 92 about a pair of pulleys 93 andfunnel 22 is supported by cables 96 mounted to pulleys 97. Theradii/diameters of the pulleys or reels may be inversely proportional tothe squares of the openings of the bunker and funnels to maintain thedesired relationship between the heights of the bunker and funnels. Thepulleys or reels 91, 93 and 97 are fixed on the shaft 87 of the liftingdevice. The cables supporting the main bunker 19 and the intermediatefunnels 21 and 22 are guided by the pulleys 98 (FIG. 13) as they arereversibly wound onto and unwound from their corresponding pulleys 91,93 and 97 to raise or lower the main bunker 19 and the intermediatefunnels 21 and 22 along the guides on the frame 83.

The size A₁₉ of the discharge opening 99 in the bottom of the mainbunker 19 is controlled by a conventional aperture plate 101 which ismoved by drive means 102 such as an electric motor. The bunker 19 servesas an intermediate storage point to accumulate material dispensed fromthe transporter 18 for loading the conveyor belt, and perhaps moreimportantly, provides a smooth uniform flow of material at a rate(volume/time) which is controlled by the distributor plate 101 to matchthe capacity of the moving conveyor belt 15.

When the speed of the conveyor belt 15 is increased/decreased, theopening A₁₉ can be enlarged/decreased by the distributor 101 to maintainequality between the dispensing rate provided by the bunker 19 and thecapacity of the moving conveyor belt 15. Thus, the volume of materialflow dispensed via the opening A₁₉ and distributor plate 101 can becontrolled to match the conveyor productivity, which is the working orcross-sectional area of the tube belt multiplied by the belt speed,P_(c) =A_(c) V_(c). The speed of the conveyor is normally much higherthan the speed of the gravity induced flow 103 of material dispensedfrom the opening A₁₉ of the bunker 19. See FIG. 13. Thus, thecross-section area A₁₉ of the opening 99 and the flow 103 must normallybe larger than the cross-section of the tubular belt 15 in order tomatch the volume of material flow at the bunker to the capacity of themoving conveyor.

The height H₁₉ and the opening A₁₉ of the bunker 19 are chosen to matchthe productivity P_(c) =A_(c) V_(c) of the tubular conveyor belt.Referring to FIGS. 1 and 13, the height H₁₉ of bunker 19 is V_(c) ² /2g.As a starting point, it is convenient to space the funnels 21 and 22equidistant between the discharge openings of the fixed funnel 23 andthe bunker. The bunker discharge opening A₁₉ is designed usingconventional methods of defining the location of the zero velocitypoint, h_(o), above this opening. The size of the bunker dischargeopening is then

    A.sub.19 =P.sub.c /(2gh.sub.o)                             (1)

where g is the acceleration due to gravity.

The cross-sectional areas A₂₁ and A₂₂ of the discharge openings of theintermediate funnels 21 and 22 are selected so that these openingsprovide a smooth laminar flow of the dispensed, falling material andsmoothly channel and compact the falling material to the openingcross-section A_(c) of the lower, fixed funnel 23 and the conveyor beltat the pulley-induced opening 24. The selected productivity P_(c) is tobe maintained at each funnel 21 and 22. That is, ##EQU1## where V_(c),V₂₁ and V₂₂ are the respective velocities of the material at theconveyor belt, the funnel 21 and the funnel 22.

Consider next the area A₂₁. Combining equations (2) and (3) gives

    V.sub.21 =P.sub.c /A.sub.21 =A.sub.c V.sub.c /A.sub.21 =2g(H.sub.19 -H.sub.21),                                               (5)

or

    (A.sub.c.sup.2 /A.sub.21.sup.2)V.sub.c.sup.2 =(2g(H.sub.19 -H.sub.21)).sup.1/2                                       (6)

and

    H.sub.19 -H.sub.21 =(A.sub.c.sup.2 /A.sub.21.sup.2 2g)V.sub.c.sup.2.(7)

Substituting H₁₉ for V_(c) ² /2g in equation (7) gives

    H.sub.21 =H.sub.19 (1-A.sub.c.sup.2 /A.sub.21.sup.2),      (8)

and

    A.sub.21 =((H.sub.19 A.sub.c.sup.2)/(H.sub.19 -H.sub.21)).sup.1/2.(9)

Similarly,

    A.sub.22 =((H.sub.19 A.sub.c.sup.2)/(H.sub.19 -H.sub.21)).sup.1/2.(10)

Thus, the height H₁₉ and area A₁₉ of the bunker are selected asdescribed above to load the material dispensed from the bunker onto theconveyor with a productivity which substantially equals the productivityof the conveyor at the given conveyor cross-sectional area A_(c) andspeed V_(c). Also, the size A₂₁ and A₂₂ of the openings of theintermediate funnels are selected as described above to create a laminarflow of material of controlled decreasing size which approximates theshape and, thus, the funneling action of a hyperboloid of revolution ofheight V² /2g and circular cross-section areas A₁₉, A₂₁ and A₂₂ at theposition of the bunker and intermediate funnel openings. The decreasingcross-sectional area of the flow of falling material is illustrated, forexample, at 103, 104 and 105 in FIGS. 13 through 15.

It should be mentioned that it may be possible to eliminate theintermediate funnels for systems which use low loading speed and height.Furthermore, the invention is not limited to the use of two intermediatefunnels to effect high speed productivity. One or more intermediatefunnels can be used. Typically, however, increasing the number ofintermediate funnels more closely approximates a continuous smoothhyperboloid funnel and thereby enhances laminar flow. Also, as theloading speed V₂₃ and the associated height H₁₉ are increased, it isadvantageous to increase the number of intermediate funnels. At present,it is believed there is no fixed optimum number of intermediate funnelsfor a particular bunker height.

Referring to FIG. 13, the cross-section of the material flow 105entering funnel 23 is just slightly larger than the opening A₂₃ of thisfunnel 23, which is approximately equal to the cross-section of theconveyor belt 15A and, thus, matches the cross-section of the flow ofloaded material to the conveyor belt. With the speed V₂₃ of the loadedmaterial also being made approximately equal to the speed of the movingconveyor belt (by choosing the height H₁₉ of the bunker to be V_(c) ²/2g), the material is loaded onto the conveyor belt at a rate A₂₃ V₂₃which equals the productivity, P_(c) =A_(c) V_(c), of the movingconveyor belt.

In addition, the curved sleeve 84 of the fixed loading funnel 23 isoriented parallel to the load branch of the belt at the opening 24 sothat the velocity (direction and speed) of the loaded materialcorresponds to that of the convey or belt. This facilitates smooth,efficient loading. In this regard, it should be mentioned that theorientation of the load section 15A of the belt is defined by the loadpulley 26 and the assembly of drive rollers at load turn 12. Theinclination of the conveyor belt can be varied by altering the positionof the end load pulley and/or the drive means.

It should be noted that the sidewalls of the funnels 21, 22 and 23 havea curved profile which can be tailored to the type of bulk material andthe nominal speed of the conveyor system. In general, it may bedesirable to use a sidewall slope which is more steep for relativelyhigh internal friction materials such as soil or coal than for lowfriction materials such as shelled corn.

The bunker 19 may be lowered during the start and stop mode of theconveyor to accommodate the reduced conveyor speed, as well as duringrelatively low productivity operation. In such cases, moving a fullyloaded belt at a reduced speed is more efficient than running apartially loaded belt at higher or maximum speed. Thus, in the lowerproductivity modes, it is desirable to reduce the speed of the fallingmaterial inside the feeder 11. This means that the bunker 19 should belowered, the material flow permitted by distributor plate 101 should bereduced, and the elevations of the funnels 21 and 22 should be lowered.In moving the bunker, the heights, H, of the discharge openings of thebunker 19 and the funnels 21,22 preferably are controlled by the use ofpulley diameters, D₉₁, D₉₃, D₉₇, which are inversely proportional to thesquares of the cross-sectional areas of the discharge openings. That is,D₉₁ :D₉₃ :D₉₇ ∝(1/A₁₉)² :(1/A₂₁)² :(1/A₂₂)². Adjustment of the height ofthe bunker 19 to control the productivity then automatically adjusts theheight of the funnels 21 and 22.

In the side and front elevation view shown in FIGS. 1 and 13, the mainbunker 19 is at its maximum height in order to provide maximumgravitational acceleration and loading speed and, thus, maximumproductivity. FIGS. 14 and 15 show the bunker at a lower height whichprovides correspondingly lower maximum productivity. The pulleydiameters D₉₁, D₉₃, D₉₇ are selected as described above so that theintermediate funnels 21 and 22 with constant discharge areas arepositioned at correspondingly lower elevations for the purpose ofmaintaining the capacity of the material flow at these elevations.

FIG. 15 depicts an insulating dust proof feeder construction whichutilizes telescoping tubings 111 and 112, 113 and 114, and 115 and 116to isolate the material flows 103, 104 and 105 from the surroundingenvironment inside the feeder tower. A, shown, the upper tube(111,113,115) of each telescoping pair is mounted to the associatedupper bunker or funnel, whereas the lower tube (112,114,116) of eachpair is mounted to the funnel at the bottom of the pair so that thetelescoping tubing pairs accommodate the variable distances between thebunker 19 and the funnels 21 and 22 and 23.

FIG. 16 shows an embodiment 11A of the feeder 11 in which the reel andcable lift system 86 is replaced by telescoping two-step and/orthree-step hydraulic jacks 117, 118 and 119. The jacks are mounted onthe intermediate floors of the feeder 11 which support the funnels 21,22 and 23. The jacks 117, 1113 and 119 can be controlled by a computeror by a hydraulic programmable device which is programmed to react tothe changing speed of the conveyor line. This embodiment does notrequire a lift device 86 and, thus, may be shorter than the feeder 11shown in FIG. 1 et al.

End Pulleys

As discussed relative to FIG. 1, the loading end pulley 26 and theunloading end pulley 2,7 are located at the opposite ends of theconveyor line for unwinding the tubular belt 15 for loading andunloading.

In the present invention, the profile of each end pulley is shaped tocontrol the path length traversed by the different sections of the beltaround the circumference of the pulley. The purpose is to equalize thelongitudinal tension in the belt as it passes over the end pulleys 26 or27. The design used to implement this goal is illustrated in FIGS. 17and 18, which schematically depict a side view (FIG. 17) and a plan view(FIG. 18) of the belt as it passes over the unloading end of pulley 27.The same principles apply to loading end pulley 26E. As shown, startingat section 1--1, the loaded branch 15A is gradually unwound as it movestoward and over the pulley 27. After unloading the bulk material at theend of its pass, the empty branch 15B is gradually wound back by theinternal prestress forces until the belt is returned to its normallyclosed tubular configuration at Section 2--2.

Two sets of points illustrate the characteristic fibers in the belt:points a₁, a₂, a₃, a₄ and a₅ are on the path traveled by a centralsection or fiber of the belt 15, whereas points b₁, b₂, b₃, b₄ and b₅are on the path traveled by an edge section or fiber. Equalization oftension in the edge and central sections requires that the paths a₁ -a₂-a₃ -a₄ -a₅ and b₁ -b₂ -b₃ -b₄ -b₅ be made equal. However, for aconventional cylindrical (flat) pulley, the distances b₁ -b₂ and b₄ -b₅are greater than the corresponding distances a₁ -a₂ and a₄ -a₅ (thedistance a₂ -a₃ -a₄ and b₂ -b₃ -b₄ are equal in a cylindrical pulley).It has been determined that, to substantially equalize stresses in thedifferent sections of belt, the edge radius r of the pulley 27 (b₂ -b₃)must be made smaller than the central radius R (a₂ -a₃) so that thepulley surface b₃ -a₃ -b₃ approximates an arc as a circle of radius a₁-a₃ and center a₁. See FIG. 18.

FIG. 19 depicts an axial view of an end pulley 26 or 27, partly insection, which is constructed to substantially equalize stresses asdiscussed above and also to substantially equalize tension in thelongitudinal belt fibers. The pulley 27 (or 26) comprises a plurality ofsegments 121-126, rotatably mounted on bearings 127 on an axle 128 whichis mounted to the framework 61. Rims 129 restrict the belt from slippingoff the roller and can be used as indicators for an electronic guidingsystem for correcting the lateral position of the belt. Radius r of edgeroller segment 126 (measured from the axle), radius R of central rollersegment 117, and the radii of the intermediate roller segments areselected so that the roller surfaces form circle arc b₃ -a₃ -b₃ ofradius a₁ -a₃ and center a₁ (FIG. 18) and the paths a₁ -a₂ -a₃ -a₄ -a₅and b₁ -b₂ -b₃ -b₄ -b₅ are substantially equal. Thus, stress in the beltis substantially equal during operation.

In addition, each roller segment 121 through 126 rotates independentlyof the others so that the large segments toward the center rotate withlower angular velocity than the smaller edge segments.

Consequently, the associated sections of the belt traverse the pulleywith substantially equal linear tangential velocities and, thus, equallongitudinal tension.

The end pulley construction shown in FIG. 19 could be incorporated intoa drive roller, if additional drive is needed at the beginning or end ofthe conveyor, by driving the individual segments 121-126 with an angularvelocity which is proportional to its circumference. This could be doneby applying a single drive through a set of gears on the axle 128 or byusing separate drive for each of the segments 121-126.

EXAMPLE

Consider now the example of the design of a high speed conveyor beltsystem supplying fuel from a distribution stockpile to a 1 megawatt coalpower plant. The following initial parameters are assumed:

Transportation distance--10 miles

Heating value of the fuel--9,500 Btu/lb

Heat rate of the fossil plant--10,500 Btu/kw-hr

Coal unit weight (size≦1 1/2")--50 lb/ft³

Fossil plant efficiency--50%.

Here, the power plant is assumed to be using one working tubularconveyor line 10 miles in length and one reserved line (100% reservedassumed). However, the high speed conveyor belt system can be employedover essentially unlimited distances, such as from coal fields in thewestern United States to a plant site, e.g., in Illinois. Also, a mainbelt system 10 can be used to feed smaller belt system 10 which conveythe coal to individual power plants.

Based upon the above initial parameters, the required coal supply is0.31 tons/sec. Assuming a conveyor belt speed of 20 meters/sec, or 44.7mph, and that the tubular belt is filled to 85% of its capacity, therequired throughput volume is 14.6 ft³ /sec. Thus, the required innerdiameter is 6.5", and the outside diameter d_(o) of the completelyclosed tubular belt is 8.0". Prestress of the belt is achieved bystraining the inner layer to 125% of its original width (5% lossesassumed, so design prestrain is 20%). The prestressed belt ismanufactured of two layers of a high elasticity latex (E=214 psi), with12 steel 1/8" .0. longitudinal wires, located in the midwall, designedto increase its stiffness as a continuous beam, supported by idlers.Bending stiffness of such a beam section is:

ΣEI=28,100 kip-in².

The self weight of the belt is:

KW_(B) =8.76 lb/f.

The weight of the coal load is:

KW_(C) =9.45 lb/f.

The distances between the idlers are determined based upon theseweights, the above stiffness and allowable deflections. Allowabledeflections are assumed to be 1/600 for a loaded branch and 1/300 for anempty one (1/100 allowable deflection is used for conventional conveyorlines) . With these assumptions, distances between support idlers arel_(L) =18.9 feet for a loaded branch and l_(E) =30.5 feet for an emptybranch. Acceptable nominal spans are l_(L) =15 feet and l_(E) =30 feet.

Based on maximum bending stress of its composite cross-section, theminimum allowable radius of conveyor line turns is R_(min) =175'.

The distributed drive is designed to fit three basic inclinations: 1)horizontal track sections; (2) 30° inclines; and 3) vertical tracksections:

1) Horizontal track. Assuming the coefficient of rolling frictionμ=0.015, the required compression between a tube and a roller is 123.7kg(f) for a loaded and 119 kg(f) for an empty branch. The roller radiusR=4"=10.2 cm. The resisting force F_(r) is estimated then at 0.18 kg(f)per idler, and the torque T=1.85 kg(f)-cm. For a one mile longhorizontal portion of a track, with 352 loaded branch idlers and 176empty branch idlers, a total torque ΣT=982 kg(f)-cm is required.

At 1872 rpm, corresponding to the conveyor belt speed of 20 μ/sec, thetotal power required for all rollers is:

H=18.88 kw/mi ˜20 kw/mi.

2) 30° incline. The self-weight of the loaded and empty branches act inopposite directions, so that only the coal weight should be considered.Note that for a coal angle of repose α=35°-45°, no sliding oftransported material inside the tubular belt is expected.

With drive idlers spaced at 30 feet apart, the torque for one idler is660 kg(f)-cm, which requires 12.7 kw power to drive per roller. Assumingthen a power requirement of 13 kw per roller, the self-weight of loadedbranch is not enough to mobilize all friction needed to produce suchtorque.

Additional compression can be achieved by pressing a satellite rolleragainst the tubular belt with a force N=109 kg(f). This force can beaccepted by a prestressed tubular belt without having it reopen (thesafety factor, with dynamic amplification kd_(A) =1.5, is k_(s) ˜4.0).On steeper slopes, the span between the drive idlers can be reduced asrequired.

3) Vertical track (90° inclination). In this case, material fills theentire inner space of the tubular belt, creating additional innerpressure on its walls. However, this pressure is limited to a maximumvalue of: ##EQU2## where ρ is the hydraulic radius, ##EQU3## γ is thecoal unit weight, and f₁ is a coal-latex friction coefficient, 0.55.

Drive idlers, driven by the same 13 kw motor as chosen above, can eachdevelop a pull-up force of 145 lbs. Then the maximum distance betweendrive idlers on a vertical portion of the track is l_(max) =13.05 feet.A nominal distance of 10 feet between drive idlers corresponds to apull-up force of about 111 lbs. A compressive force N=370 lbs. is thusneeded between an idler (and satellite) and the belt to pull up theconveyor line on a 90° vertical track. This provides a safety factor of˜3.0 for a tubular belt configuration.

From the above description and examples of the various presentlypreferred and alternative embodiments, those of usual skill in the artwill readily derive, modifications of the present invention which arewithin the spirit and scope of the invention as defined by the claims.

What is claimed is:
 1. A method of forming a prestressed normally closedtubular conveyor belt, comprising the steps of:providing a firstelongated layer of elastic material having a predetermined width;stretching a second, relatively narrow elongated layer of elasticmaterial to substantially the width of said first layer by applying,tensile force transverse to the length of said second layer; aligningsaid two layers, joining said layers together while maintaining tensionon said second layer, whereby upon releasing the applied tension aftersaid joining, the joined layers maintains the first layer in tensionalong its width and the second layer in compression along its widthsufficient so that the resulting bending moment transversely bends thejoined layers along its width to a normally closed tubular configurationhaving a tubular angle >=360°.
 2. A normally closed tubular conveyorbelt having width and length and a longitudinal axis, comprising:elastic inner and outer layers joined together with the inner layer intension along its width and the outer layer in compression along itswidth; and a plurality of rollers spaced apart along the longitudinalaxis of the belt, for supporting the belt, the tensile and compressiveforces being sufficient so that the resulting internal bending momentprovides an inherent curving effect normally bending the belt along itswidth to a substantially tubular angle ≧360°, and vertical stiffness ofthe belt determining the distances along the longitudinal axis betweenthe support rollers.
 3. The conveyor belt of claim 2, wherein the beltincludes reinforcing means for increasing stiffness thereof along aselected diameter, and for securing the closed tubular belt againstrotation around said longitudinal axis comprising a pair of first andsecond relatively thick longitudinally extended belt sections onopposite sides of the belt at said diameter; said conveyor belt furthercomprising: spring biasing means mounted to or within the belt,comprising (1) a multiplicity of circumferential spring bands havinginherent spring effect for normally bending to a substantially tubularangle ≧360°; and (2) first and second longitudinally extending edge ribelements mounted within each of said relatively thick sections alongsaid diameter.
 4. A composite two layer conveyor belt having a width, athickness and a length and a longitudinal axis extending along saidlength, said composite two layer belt having longitudinally extendingbelt edges comprising: elastic inner and outer layers joined togetherwith the inner layer in tension along its width and the outer layer incompression along its width, the tensile and compressive forces beingsufficient so that the resulting internal bending moment provides aninherent curving effect normally bending the composite two layer belt toa substantially tubular angle >=360°, along its width and maintains thebelt in a normally closed tubular configuration with the longitudinalextending belt edges in butt connection across the thickness of the beltconstantly under compressive stress against one another, a springbiasing means mounted to or within the belt, said spring biasing meanscomprising:a multiplicity of circumferential spring bands havinginherent springing effect for normally bending to a substantiallytubular angle >360°, and a relatively rigid longitudinal rib supportingthe individual bands; and a pair of relatively rigid longitudinal ribsjoined to the opposite longitudinal ends of the array of circumferentialspring bands.
 5. The conveyor belt of claim 4, wherein one of thelongitudinally extending edges forms a groove and the otherlongitudinally extending edge forms a tongue to lock into said groove.6. A conveyor belt prestressed along its transverse width to provide anormally closed tubular configuration, comprising:at least one layer offlexible belt material; and spring biasing means mounted to or withinthe belt layer, comprising: a multiplicity of circumferential springbands, each having first and second ends and having inherent springingeffect for normally bending to a substantially tubular angle ≧360°;first and second relatively rigid longitudinal ribs joined,respectively, to said first ends and to said second ends of thecircumferential spring bands; and a third relatively rigid longitudinalrib mounting said bands intermediate said first and second ends.
 7. Aconveyor belt in combination with a system for driving adjacent load andreturn branches of said belt, the conveyor belt comprising:elastic innerand outer layers joined together with the inner layer in tension alongits width and the outer layer in compression along its width, thetensile and compressive forces being sufficient so that the resultinginternal bending moment provides an inherent curving effect normallybending the belt along its width to a substantially tubular angle ≧360°;and the system further comprising: a drive roller positioned between thebranches of said belt for rotatably engaging and driving the branches; apair of first and second idler rollers; and means adjustably supportingthe first and second idler rollers, respectively, on the load and returnbranches of said belt on opposite sides thereof from the drive rollerfor varying the force of engagement of said rollers with said belt.